User equipment (UE) to network relay (UE-to-Network Relay) is a hot topic in the Proximity Service Enhancement work item of the 3GPP (3rd Generation Partnership Project) R13. By controlling a remote UE to communicate with a base station via a relay UE, the network coverage can be extended, the transmission rate of a UE at an edge of a cell can be increased, and the spectrum reusability can be improved. Therefore, it is a prospective technology of the LTE-Advanced system.
At present, discovery between UEs in a proximity service may be performed in the following modes.